In the current state of the art, separate networks are employed for data terminals which are parallel to networks for voice communication. Typically, display screens and control units are connected by a network of coaxial cables. The control units are connected directly, or via modems and analogue telephone lines, to a network node or a front-end computer. There is a separate network for voice communication parallel to these networks, which in some cases consist of several parallel networks for different computer systems.
New types of networks have been proposed for speech and data to provide integrated communication, above all in large office environments. Examples of these are high speed networks (local area networks or LAN) proposed by suppliers of data and office automation equipment, and new types of telephone exchanges (PABX) have been proposed for digital switching and transmission by suppliers of telephone equipment.
The LAN solutions are mainly directed to data traffic with "all-to-all communication" without physical switching. The PABX solutions are characterized by being circuit switched to give the opportunity for coupling the connected apparatus together in pairs. The apparatus can relate either to telephones or data equipment. In modern PABX's data and speech can be transferred simultaneously and mutually independently through the same cabling. In certain cases, a PABX may also be made arranged as a physically distributed system kept together by multiplexed high-speed connections, providing communication ability over large geographic areas in the same way as within a building.
Examples are given below of some of the problems existing in the data communication field, above all with large terminal densities.
From the administrative aspect it is impractical, in the state of the art, to utilize two or more parallel communication networks with completely different administrative routines ("catalogue services" etc). This is the case with today's systems (coaxial networks) as well as in the LAN solutions discussed. The circuit-switched PABX solutions proposed to date can partially solve these problems, but with higher cost and with less flexibility than in the solution provided in accordance with the invention.
In an environment where the terminal system is rapidly changed by additions and removals, the administrative routines often constitute a restraining factor with regard to development. In the present state of the art, the terminals are tied together with a permanent coaxial network to a control unit. When a terminal is moved, new cabling must be run and documented, and the host computer to which the terminal is to be connected must have its program altered (network generation). With larger terminal networks, the latter requires a very large data programming force, which is costly as well as being so demanding that it can only be carried out infrequently. In turn, this results in long delays when there are alterations, i.e. installed terminals cannot be used.
The possibilities of improved availability are also exemplified by the case where many (typically about 30) terminals are connected to one control unit.
Should there be a fault in the control unit, its line or modem, all the terminals lose contact with the host computer.
Faults' locations may be determined either by physically moving the modem cable on the host computer side to a testing center, by different test procedures from the host computer or by local fault locating at the control unit.
In modern solutions (pertaining to coaxial networks, etc.) there is a fixed connection between control units and terminals, and also generally between the control unit and the computer. This means that in workplaces where access to several computer systems is required, several terminals must be connected to different control units. This also applies if the terminal systems are programmable for enabling connection to different computer systems, since all terminals related to a control unit must change function simultaneously.